Putting it into a board you've never ridden before, you really won't feel any difference between a tunnel fin and a normal fin. I put one in an 8'6 I had for a long time that I used to ride a cutaway in and the difference was that the tunnel felt stiffer and more stable... almost tracky in the tail... and possibly slightly faster. One session and I knew it wasn't for me.
The shaper most-likely designed the board with a fin in mind. If you're getting a used board that's missing the original fin and can't find out what fin it came with, go with an 8" or 9" Greenough 4A. Just my 2 cents.
Yea thanks guys for the info I ended up getting a "normal" center fin(8in) then got the normal sized side bites. I was just curious shot those tunnel fins because I've always seen them on eBay but never in person so I was wondering if they were someones backyard project..lol